1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to a mask and a method of fabricating a mask used in extreme ultraviolet lithography (EUVL).
2. Discussion of Related Art
Continual improvement in photolithography has allowed the shrinkage of semiconductor integrated circuits (IC) to achieve ever higher density and performance. Deep ultraviolet (DUV) light with a wavelength of 248, 193, or 157 nanometers may be used for optical lithography. However, a paradigm shift to more exotic technologies is inevitable. Viable contenders for Next Generation Lithography (NGL) include electron projection lithography (EPL), ion projection lithography (IPL), x-ray projection lithography (XPL), and extreme ultraviolet lithography (EUVL).
EUVL is a leading candidate for NGL, especially for fabrication of high volume ICs. Exposure is performed with extreme ultraviolet (EUV) light with a wavelength of about 10-15 nanometers. EUV light falls in a portion of the electromagnetic spectrum referred to as soft x-ray (2-50 nanometers). Whereas a conventional mask used in DUV lithography is made from fused quartz and is transmissive, virtually all condensed materials are highly absorbing at the EUV wavelength so a reflective mask 180, as shown in FIG. 1, is required for EUVL.
An EUV step-and-scan tool typically uses a 4X-reduction projection optical system. A wafer is exposed by stepping fields across the wafer and scanning an arc-shaped region of the EUV mask for each field. The EUV step-and-scan tool may have a 0.10 Numerical Aperture (NA) with 4 imaging mirrors and 2 collection mirrors. A critical dimension (CD) of 50-70 nanometers may be achieved with a depth of focus (DOF) of about 1 micrometer. Alternatively, the tool may have a 0.25 NA with 6 imaging mirrors to print a smaller CD, such as 20-30 nanometers, but the DOF will be decreased significantly. Other tool designs, including 5X, 6X, and 10X reduction, may be used.
The variability in CD printed on a wafer with EUV lithography depends strongly on the absorber height 603 on the EUV mask 680, as shown in FIG. 6. An oscillating relationship results from interference between the light 606 reflected off the multilayer (ML) mirror within the blank areas of the mask 680 and the light 608 reflected off the upper surface of the mask absorber. The phase difference between the principal light rays oscillates with half the wavelength of the incident light. Constructive and destructive interference occurs for absorber heights 603 differing by only a quarter of a wavelength or about 3 nanometers. Such a variation in absorber height 603 of 3 nanometers will cause CD on a wafer to vary by approximately 4 nanometers.
Wafer CD variation can potentially be minimized by controlling the variation in the thickness of the mask absorber 660 within the entire mask 680 to less than 3 nanometers. However, existing deposition tools are not able to deliver such a tight uniformity for the thickness of the mask absorber 660.
Thus, what is needed is a reflective EUV mask to produce tight CD control on a wafer and a process for fabricating such a reflective mask.